Flora family

The Flora or Florianfamily of asteroids is a large grouping of S-typeasteroids in the inner main belt, whose origin and properties are relatively poorly understood at present. Roughly 4-5% of all main belt asteroids belong to this family.

Because of its poorly defined boundaries, and the location of Flora itself near the edge, this asteroid group has also sometimes been called the Ariadne(an) family, when Flora did not make it into the group during an analysis (e.g. the WAM analysis by Zappala 1995).

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The largest member is 8 Flora, which measures 140 km in diameter, and comprises about 80% of the total family mass. Nevertheless, the parent body was almost certainly disrupted by the impact/s that formed the family, and Flora is probably a gravitational aggregate of most of the pieces.[citation needed]43 Ariadne makes up much of the remaining mass (about a further 9%), with the remaining family members being fairly small, below 30 km in diameter.

A noticeable fraction of the parent body has been lost from the family since the original impact, presumably due to later processes such as e.g. secondary collisions. For example, it has been estimated that Flora contains only about 57% of the parent body's mass (Tanga 1999), but about 80% of the mass in the present family.

The Flora family is very broad and gradually fades into the background population (which is particularly dense in this part of space) in such a way that its boundaries are very poorly defined. There are also several non-uniformities or lobes within the family, one cause of which may have been later secondary collisions between family members. Hence, it is a classical example of a so-called 'asteroid clan' (see asteroid family). Curiously, the largest members, 8 Flora and 43 Ariadne, are located near the edge of the family. The reason for this unusual mass distribution within the family is unknown at present.

951 Gaspra, a medium-sized core family member was visited by the Galileo spacecraft on its way to Jupiter, and is one of the most extensively studied asteroids. Studies of Gaspra suggests that the family's age is of the order of 200 million years (indicated by the crater density), and that the parent body was at least partially differentiated (indicated by the high abundance of olivine) (Veverka 1994).

The Flora family members are considered good candidates for being the parent bodies of the L chondrite meteorites (Nesvorny 2002), which contribute about 38% of all meteorites impacting the Earth. This theory is supported by the family's location close to the unstable zone of the secular resonance, and because the spectral properties of family members are consistent with being the parent bodies of this meteorite type.

The Flora family was one of the five original Hirayama families that were first identified. It has a high number of early discovered members both because S-type asteroids tend to have high albedo, and because it is the closest major asteroid grouping to Earth.

Zappala's 1995 analysis found 604 core members, and 1027 in a wider group. A search of a recent proper element database (AstDys)for 96944 minor planets in 2005 yielded 7438 objects lying within the rectangular-shaped region defined by the first table above. However, this also includes parts of the Vesta and Nysa families in the corners so that a more likely membership estimate is 4000-5000 objects (by eye). This means that the Flora family represents 4-5% of all main belt asteroids.